The invention relates to a method of cooling rotary spindles and/or other sensitive parts of machines, e.g., to a method of cooling the spindle and the antifriction bearing or bearings in a milling machine, a boring or drilling machine or another machine tool. The invention also relates to improvements in machines wherein the spindles and/or other parts are cooled in accordance with the novel method.
Adequate cooling of rotary parts of certain recent types of machines, for example, adequate cooling of spindles and antifriction bearings in modern milling or boring machines and/or certain other machine tools, presents numerous problems. Accumulation of substantial quantities of heat in the column or head of a machine wherein the spindle and its bearings are mounted in the head can result in undesirable thermally induced expansion of the spindle with attendant reduction of the accuracy of finish of the workpieces. Moreover, overheating of the spindle and of its bearings can affect the ability of the machine to repeatedly treat workpieces, including shorter or longer series of successively treated workpieces, with the same degree of accuracy and reproducibility.
In many conventional machines wherein the spindle is driven by an electric motor through one or more gear transmissions or other types of transmissions, the head or column of the machine as well as the spindle and its bearings are cooled by oil or another lubricant which is used to lubricate the bearing or bearings and the transmission or transmissions as well as to withdraw heat from the column and from the spindle. The withdrawal of heat from the column and from the spindle is effected by a heat exchanger which causes the confined supply of oil to exchange heat with air or with another coolant, such as Freon (trademark). The heat exchanger contributes to the bulk and complexity as well as to the initial and maintenance cost of the cooling system and of the entire machine. Moreover, cooling systems which employ oil and means for effecting an exchange of heat between oil and air or another fluid coolant are satisfactory only when the rotational speed of the spindle does not exceed a relatively low value, such as approximately 3000 RPM.
Certain recent types of machines employ spindles which receive torque directly from high-speed electric motors capable of rotating the spindles at speeds of up to and even in excess of 4000 RPM. Such motors are often equipped with electronically operated speed regulating means which can rapidly accelerate the rotor of the motor and the spindle of a machine to a speed of at least 4000 RPM. The establishment of direct connection between the output element of a motor and the spindle of a milling, boring or other machine renders it possible to dispense with the gear transmission(s) and/or with other types of transmissions which are used in heretofore known machines to transmit torque from the motor to the spindle. This, in turn, renders it possible to dispense with means for lubricating and cooling the transmission or transmissions. Thus, if such machines continue to employ supplies of lubricant, the lubricant is used solely to perform a cooling action. It was believed that such cooling and lubricating means cannot be omitted because a relatively long spindle which is mounted in several antifriction bearings to rotate in the housing of a machine tool at speeds of up to 4000 RPM invariably causes the development of localized overheating, especially in the regions of the antifriction bearings. The bearings are likely to be overheated even if they contain encapsulated supplies of grease or another lubricant, i.e., even if the bearings are of the type requiring no outside lubrication in actual use. It has been found that bearings which contain encapsulated supplies of grease or another lubricant are still likely to be overheated in a machine wherein they are called upon to surround portions of a spindle or an analogous rotary part which is driven at speeds of up to and even in excess of 4000 RPM. Thus, it is necessary to adequately cool the spindle and the bearings in a machine wherein the bearings contain encapsulated supplies of grease or another lubricant and the spindle receives torque directly from a motor, i.e., without the interposition of one or more transmissions which employ gears and require continuous or frequent lubrication. If the cooling is effected by resorting to a liquid, a supply of such liquid must be confined in and circulated within the housing for the sole purpose of preventing overheating of the housing, bearings and spindle, i.e., such liquid need no longer perform any lubricating action. The installation of liquid-containing cooling systems in a machine tool or in another machine contributes excessively to the bulk and complexity as well as to the initial and maintenance cost of such machines.